The present invention relates to radio communications technology and more particularly to receiver/exciter technology.
At the present time the vast majority of radio receivers are of the superheterodyne type employing one or more intermediate frequency stages which allow for filtering and amplification at fixed frequencies. Alternatives exist to the superheterodyne architecture such as the superregenerative receiver, the direct conversion receiver, and the direct sampling receiver. However, these alternative designs have been subject to serious flaws which have relegated radio receivers of these types to specialty roles within the radio communications world.
A typical heterodyne receiver is comprised of translators that perform one or more conversions of the received signal to an intermediate frequency (IF) that can then be more readily processed for information content of the carrier wave. The heterodyne receiver requires the use of one or more local oscillators and associated filters, amplifiers and mixers to process the received signal. Undesired spectral signals or spurs can corrupt the processing of the desired signal. Spurs such as a mixer image signal or higher order product must be countered in order to process the desired signal. The intermediate frequencies chosen and the filtering at each translation determine the level of rejection of spurious signals.
The mixer circuit in a superheterodyne radio receiver translates a signal received at one frequency to another frequency, termed an intermediate frequency (IF). Undesired signals at the mixer input can also be translated onto the same intermediate frequency. These undesired signals or spurs can be located at the image frequency of the mix or at frequencies whose higher order products of the non-linear mix produce the same intermediate frequency as the desired signal. Superheterodyne receivers are typically designed with multiple conversions in which each conversion is protected by a filter and avoids spurious signals. The final conversion produces an intermediate frequency that can be demodulated. High performance superheterodyne receivers may require four or more conversions with associated filtering and amplification to reject spurious signals.
Partly in order to avoid the above described problems, designers have utilized direct conversion receivers. While direct conversion receivers avoid the multiple conversion problems of superheterodyne receivers other drawbacks exist. One such type of problem is described as local oscillator leakage. Since the local oscillator and the tuned frequency are the same, the receiver is unable to filter the local oscillator signal. To minimize the effects of oscillator leakage designers have relied upon the use of additional components, like amplifiers, in reverse isolation fashion in order to minimize its effects. Other problems such as direct current offsets, hum susceptibility, microphonics, quadrature I and Q balance, and direct undesired amplitude modulation detection are also present in typical direct conversion receivers. Accordingly, a need exists for a simplified approach to signal processing that eliminates the drawbacks of the prior art solutions.